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    <ul>
        <li>山有木兮木有枝</li>
        <li>心悦君兮君不知</li>
        <li>弃我去者,昨日之日不可留</li>
        <li>乱我心者,今日之日多烦忧</li>
    </ul>


    <ol id="ol">
        <li>落花有意随流水</li>
        <li>流水无心恋落花</li>
        <li>力拔山兮气盖世</li>
        <li>时不利兮气不锥</li>
    </ol>


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        //  根据标签获取元素的(伪)数组
        let result = dicument.getElementByTagName('li');
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        //2.伪数组,具有数组的结构,但是不能使用数组的方法.
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//result.push('a')


//3.获取元素内部的标签,指定父级
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